In 5.6.1, the above throws warnings for FOO and DIR, and all of the our
variables, but nothing else --- in 5.8.0, only FOO and DIR elicit
warnings. Variables declared with 'use vars' (and now our) are
exempt, as are subroutines and lexicals.

To top it off, 'used only once' really only applies to things in the
symbol table, and in particular, is elicited only when the *typeglob
is used but once --- as soon as two or more slots in a typeglob are
filled, all identifiers within that typeglob are exempt:

#!/usr/bin/perl -w
use strict;
use vars '$foo';
our(@foo); # second use of *foo
$main::bar = 42;
%main::bar = (a => 1); # second use of *bar
our($FOO, @DIR);
open(FOO,$0) or die $!; # second use of *FOO
opendir(DIR, '.') or die $!; # second use of *DIR
sub blah {}
our($blah); # second use of *blah